We report a case of a patient, AD, who suffered from acalculia following an infarct restricted to the left IPS (intraparietal sulcus). AD showed deficits in an arithmetic battery and in basic numerical processing tasks. Compared to matched controls, AD showed a larger distance effect in numerical comparisons, an abnormal size congruity indicated by a lack of facilitation by irrelevant numerical values in physical comparisons, a deficiency in counting in comparison tasks, and a deficiency in subitizing in number naming tasks and comparison tasks. This suggests that the underlying deficit in AD is a difficulty in the perception and manipulation of quantity. Moreover, it indicates the essential role of the IPS in basic numerical processing.